Japanese mobile operator SoftBank says it will acquire a 70 percent stake in Sprint for a total of $20.1 billion. The deal represents Sprint’s best chance at staying relevant as a nationwide carrier and leaves AT&T as the sole large, U.S. carrier without a large international owner.

SoftBank is acquiring the stake in part to take advantage of Sprint’s spectrum — the nation’s No. 3 carrier is in the midst of a nationwide LTE deployment — as well as to advance SoftBank’s own growth in the saturated Japanese market. And Sprint needs the cash. The company has made bet after bet that haven’t panned out — from the ill-fated Nextel acquisition to the all-in investment on the iPhone.

The deal will consist of $8 billion in cash for Sprint and $12.1 billion paid to Sprint shareholders. It’s expected to close during the middle of 2013 pending regulatory approval. Sprint has upwards of $21 billion in debt with some of that maturing next year. However, buying a stake in Sprint as a means to pay down debt it a pretty crappy use of cash, so one hopes that Sprint will use of the dollars to acquire spectrum — either through consolidating more control over Clearwire, or perhaps in an upcoming spectrum auction.

So the deal is a positive for Sprint, and perhaps might help SoftBank by giving it a foothold in the U.S. market, but it raises several questions about the state of mobile play in the U.S. While this deal addresses Sprint’s need for cash, it doesn’t address a fundamental question for the U.S. market– namely does it need four nationwide carriers?

As of 2011 when AT&T was proposing its T-Mobile buy, the U.S. was one of the most competitive markets. Is that sustainable?

Also this month, T-Mobile said it would acquire Metro PCS, a pre-paid provider for $1.5 billion, after T-Mobile failed to close an earlier merger with AT&T. At the heart of these ownership struggles is the cold hard fact that the U.S. market is dominated by AT&T and Verizon and those players have carved out a huge spectrum advantage. They also own much of the backhaul network that connects cell towers back to the internet, which means that both Sprint and T-Mobile have to pay their mobile rivals for fiber backhaul.

So for the last two years, we’ve had to endure any number of possible combinations that would bring the U.S. carriers down to what some analysts view as a more rational number. There have been rumors of T-Mobile and Sprint getting together. Variations on those two buying Leap Wireless or Metro PCS and the aforementioned AT&T and T-Mobile deal.

With this deal and the T-Mobile acquisition of Metro PCS, we’re no closer to getting the U.S. market to three operators, so the real question after this deal, is still going to be whether the U.S. can support four operators as the need for network investment and spectrum escalates to meet our demand for mobile data.